Treating boron-containing steel



Patented May 14, 1946 2,400,302 TREATING BORON-GONTAINING STEEL Raymond Andrew Grange, Jersey City, N. 1., as-

signor to United States Steel Corporation of Delaware, a corporation of Delaware No Drawing. Application July '16, 1943, Serial No. 495,027

1 Claim.

This invention is concerned with plain-carbon and low alloy, hypoeutectoid steel containing boron, in the small amounts used to improve the steel's hardenability and structure, the invention being a treatment providing, in such steel, a microscopically visible precipitate of a boron constituent within the steel's free ferrite. This is useful in that the continuity of this ferrite is interrupted, so as to enhance the steels machinability, and in that it becomes possible to metallographically determine the presence of boron, which has not been heretofore possible. This element is used very sparingly in steel, it preferably ranging from .0005 to .005%.

As indicated, the invention is a method of treating boron-containing steel having a normal structure including free ferrite, this meaning ferrite not tied up with the carbide as lamellar pearlite, such a steel being representative of any plaincarbon and low alloy, hypoeutectoid steel con-.

taining boron. I

This method comprises heating the steel to provide it with austenite of relatively large grain critical, downwardly, time must be allowed for the precipitate to develop, but. as has been inferred, it is, generally speaking, immaterial whether this is done by holding the steel at a temperature slightly below its critical, or by cooling with sui'ficient slowness. The precipitate has the appearance of fine strings of beads, extending through the free ferrite, outlining what were the austenite grain boundaries.

iAustenitizing temperatures of from 1800 to 2200" F. are usually adequate to coarsen the austenite grains, as required to carry out this invention, and rapid cooling to 1100 to 1300 E, followed by holding or slow cooling, is suillcient to obtain the visible precipitate of the boron constituent within the free ferrite grains. The precipitate enhances machinability, since it interrupts the continuity of the ferrite which is, otherwise, so ductile as to interfere with machining.

In addition, the development of the precipitateprovides a means for determining, metallographically, the presence of boron in the steel.

1 claim:

A method of treating plain carbon and lowalloy hypoeutectoid steel containing from about .0005 to .005% boron and having a normal structure including free ferrite to provide a microscopically visible precipitate of a boron constituent suflices to develop a visible precipitate of the boron constituent. The exact nature of this constituent is not now known, but it is easily visible and is not to be found when boron is absent from the composition of the steel. After passing the 4 within the steel's free ferrite, said method comprising heating the steel to from about 1800' to 2200 F. to provide it with auatenite grains of at least No. 4 A. S. '1. M. size, quenching the steel to from about 1100 to 1300 F. and at least to the A; critical temperature of the steel and after passing this temperature downwardly allowing time for said precipitate to develop by maintaining the steel at temperatures slightly below said temperature.

RAYMOND vsrmnsw GRANGE. 

